Nuclear rockets could travel to Mars in half the time − but designing the reactors that would power them isn't easy

[Admin]

Fission technology is well established in power generation and nuclear-powered submarines, and its application to drive or power a rocket could one day give NASA a faster, more powerful alternative to chemically driven rockets.

Nuclear rockets could travel to Mars in half the time − but designing the reactors that would power them isn't easy : Read more

 

[Unclear Engineer]

It would be a lot easier if they just went ahead and used highly enriched uranium for the reactor. These reactors are not going to be used where they could be stolen by non-nuclear states so that they could lead to "proliferation" of atomic weapons in currently non-nuclear countries.

And, anyway, the countries like North Korea and Iran are developing nuclear weapons anyway, as did India and Pakistan.

So, I do not see any realistic reason to not use the best possible materials for space exploration vehicles. We aren't going to return these thing to Earth's surface once they have gone critical and produced power and radioactive byproducts.

 

[Passacaglia]

Most likely reaction mass will be hydrogen (as shown in the diagram) and not oxygen (as stated in the text).

 

[newtons_laws]

It would be a lot easier if they just went ahead and used highly enriched uranium for the reactor. These reactors are not going to be used where they could be stolen by non-nuclear states so that they could lead to "proliferation" of atomic weapons in currently non-nuclear countries.

And, anyway, the countries like North Korea and Iran are developing nuclear weapons anyway, as did India and Pakistan.

So, I do not see any realistic reason to not use the best possible materials for space exploration vehicles. We aren't going to return these thing to Earth's surface once they have gone critical and produced power and radioactive byproducts.

Your argument has a lot of merit. I believe the HALEU fuel being considered has about 20% U235 enrichment.

 

[fj.torres]

The time for nuclear thermal was last century.

By the time the politics align to put fission reactors into space (2040?) newer tech will be viable, ranging from MHD-enhanced exhaust to plasma engines. Like a VASIMR derivative driven by a reactor instead of solar or fuel cell. And hydrogen is finicky and low mass. Better exhaust materials can be found, especially if lunar sourced. In fact, if He3 is actually mine-able on the moon, a fusion drive might be economically viable.

Where fission will be most useful is in providing electrical power for the truly advanced drives. Say something like the Westinghouse 5MW eVinci. And that is still a thermal-based tech. If any of the small fusion power concepts do pan out, even more exotic options would be doable.

It may be best to stop trying to undo the mistakes and bad choices of the past and look instead to the technologies that are emerging for both power and propulsion...
...while remembering there will still be a need for cheap chemical vehicles for landers and such. It is not too fanciful to conceive of a large advanced transfer vehicle with a dozen Starship-class ships attached to hold cargo and passengers.

Fast propulsion systems are necessary but you have to get off the mudball first and most advanced concepts are unlikely to provide the thrust needed for that. Until the ivory tower designs can get off planet on their own there will still be a need for chemical.

 

[Meteoric Marmot]

I believe the issue with enhanced uranium is that it violates weapons treaties and can't be produced. I'm going from memory here, so I could be wrong.

 

[billslugg]

The permanent members of the Security Council can enhance uranium, no one else who signed NNPT can. The five are obligated to provide the signatories with reactor fuel. Those countries did not sign NNPT, which is Israel, Pakistan, India and South Sudan can do whatever they want but they can also be "first targeted" legally.

 

[bolide]

It would be a lot easier if they just went ahead and used highly enriched uranium for the reactor. These reactors are not going to be used where they could be stolen by non-nuclear states so that they could lead to "proliferation" of atomic weapons in currently non-nuclear countries.

And, anyway, the countries like North Korea and Iran are developing nuclear weapons anyway, as did India and Pakistan.

So, I do not see any realistic reason to not use the best possible materials for space exploration vehicles. We aren't going to return these thing to Earth's surface once they have gone critical and produced power and radioactive byproducts.

The 'proliferation' problem is not by any means limited to non-nuclear states trying to produce nuclear bombs. The stuff still has to be produced, handled and transported on the ground. Any sort of bad actor gaining any sort of control over any amount of HEU could create catastrophic risk.

 

[bolide]

I wonder why they choose the lightest possible substance for the propellant. Something heavier should provide more thrust for a given exhaust velocity, or require a lesser velocity for the same thrust. It would also take up less space.

 

[Classical Motion]

I have read some unbelievable things about metal foams. Like that within the same volume, a foam can hold more molecules than a vacant volume under our highest tank pressures.

And the molecules are under very low pressure. And the foam under very low tenser stress.

That’s pretty amazing.

The big breakthroughs in the future will be with structure, not mass and energy.

The conversion and ratio of mass and energy is set and varied with a physical structure.

 

[Unclear Engineer]

A coupe of comments:

1. The countries with nuclear weapons have the capabilities and rights under the treaty to produce weapons-grade enriched uranium. And they all need to continue to do so to maintain their nuclear warheads, which have continuous natural nuclear decay going on that can degrade them. So, the issue is commercial use of weapons-grade material, not having it, and treaties against "weapons in space", which a rocket motor really is not.

2. Regarding using light weight atoms (or molecules) for the highest efficiency in producing rocket thrust, the reason is that rocket thrust is based on the momentum of the exhaust gas, while the speed of the exhaust gas depends on the amount of energy that can be put into it.

With a nuclear rocket, the energy is not dependent on the chemistry of the propellants, so the choice of exhaust gases is free of the need for it to be heated by burning it.

So, for example, if you apply enough energy to push an atom with atomic mass 1 to a velocity of 1, that amount of energy is 1/2 x mass x velocity squared which is 0.5 for those values.

Now consider a single exhaust gas molecule with a mass of 2. Assuming the same amount of energy in that exhaust particle and solving for its velocity, that particle will have a velocity equal to the square root of (0.5 divided by1/2 and divide by 2) = square root of 1/2 [instead of the square root of 1] = 0.71

Now, look at the momentum for the same amount of energy in the two particles of different masses. Momentum is proportional to the velocity, not the square of the velocity.

So, the particle with mass 1 and velocity 1 has momentum of 1, while the particle with mass 2 has momentum equal to 2 times 0.71 = 1.42. So, it seems like the heavier particle provides more thrust for a given amount of energy per particle. But, not for a given mass of propellant, which is the concern for rocket vehicle performance.

The efficiency ("specific impulse" abbreviated Isp) of a rocket motor is the amount of thrust per unit mass of the propellant used per unit time. So for the particle with mass 1 being used one per second, the Isp is 1.0 divided by one per second, which equals 1 second. The particle with mass 2 being used 1 per second has an Isp of 1.42 divided by 2 equals 0.71. so, the efficiency of the heavier particle is lower by 29%.

To really appreciate the significance of the Isp efficiency, think of it as the total amount of "impulse" that it can produce, which is the amount of force multiplied by the length of time it can be produced. So, for the 2 propellant particles of different masses, the lighter particle with an Isp of 1 second would produce a total "impulse" of 1 times 10 =10 for 10 seconds of propellant flow, while the heavier particle with the same mass flow rate would only produce 0.71 times 10 = 7.1 , because the same mass flow rate of the heavier propellant is 1/2 the particle flow rate of the lighter propellant.

So, for a given total mass of propellant, the lighter exhaust particles provide more total thrust over the same period of time that the rocket engine runs.

That is the basic physics, but the engineering is not so simple. For one thing, the weight of the equipment needed to create the propellant exhaust velocity is much different between a chemical rocket and a nuclear thermal rocket, which needs a whole, heavy nuclear reactor and probably some shielding that a chemical rocket does not have. And, there are also some other issues about where the energy from the reactor can go besides the velocity of the propellant particles. For instance, if the propellant particle is a molecule instead of a single atom, when it is heated, some of the energy goes into either vibrational and rotational states of the atoms within the molecule, or into separation of those atoms by breaking the chemical bond between them.

So, in reality, the benefits of a nuclear rocket vs a chemical rocket need to be calculated between 2 actual designs, one for each, to determine the real difference for specific mission requirements.

 

[bolide]

A coupe of comments:

1. The countries with nuclear weapons have the capabilities and rights under the treaty to produce weapons-grade enriched uranium. And they all need to continue to do so to maintain their nuclear warheads, which have continuous natural nuclear decay going on that can degrade them. So, the issue is commercial use of weapons-grade material, not having it, and treaties against "weapons in space", which a rocket motor really is not.

2. Regarding using light weight atoms (or molecules) for the highest efficiency in producing rocket thrust, the reason is that rocket thrust is based on the momentum of the exhaust gas, while the speed of the exhaust gas depends on the amount of energy that can be put into it.

With a nuclear rocket, the energy is not dependent on the chemistry of the propellants, so the choice of exhaust gases is free of the need for it to be heated by burning it.

So, for example, if you apply enough energy to push an atom with atomic mass 1 to a velocity of 1, that amount of energy is 1/2 x mass x velocity squared which is 0.5 for those values.

Now consider a single exhaust gas molecule with a mass of 2. Assuming the same amount of energy in that exhaust particle and solving for its velocity, that particle will have a velocity equal to the square root of (0.5 divided by1/2 and divide by 2) = square root of 1/2 [instead of the square root of 1] = 0.71

Now, look at the momentum for the same amount of energy in the two particles of different masses. Momentum is proportional to the velocity, not the square of the velocity.

So, the particle with mass 1 and velocity 1 has momentum of 1, while the particle with mass 2 has momentum equal to 2 times 0.71 = 1.42. So, it seems like the heavier particle provides more thrust for a given amount of energy per particle. But, not for a given mass of propellant, which is the concern for rocket vehicle performance.

The efficiency ("specific impulse" abbreviated Isp) of a rocket motor is the amount of thrust per unit mass of the propellant used per unit time. So for the particle with mass 1 being used one per second, the Isp is 1.0 divided by one per second, which equals 1 second. The particle with mass 2 being used 1 per second has an Isp of 1.42 divided by 2 equals 0.71. so, the efficiency of the heavier particle is lower by 29%.

To really appreciate the significance of the Isp efficiency, think of it as the total amount of "impulse" that it can produce, which is the amount of force multiplied by the length of time it can be produced. So, for the 2 propellant particles of different masses, the lighter particle with an Isp of 1 second would produce a total "impulse" of 1 times 10 =10 for 10 seconds of propellant flow, while the heavier particle with the same mass flow rate would only produce 0.71 times 10 = 7.1 , because the same mass flow rate of the heavier propellant is 1/2 the particle flow rate of the lighter propellant.

So, for a given total mass of propellant, the lighter exhaust particles provide more total thrust over the same period of time that the rocket engine runs.

That is the basic physics, but the engineering is not so simple. For one thing, the weight of the equipment needed to create the propellant exhaust velocity is much different between a chemical rocket and a nuclear thermal rocket, which needs a whole, heavy nuclear reactor and probably some shielding that a chemical rocket does not have. And, there are also some other issues about where the energy from the reactor can go besides the velocity of the propellant particles. For instance, if the propellant particle is a molecule instead of a single atom, when it is heated, some of the energy goes into either vibrational and rotational states of the atoms within the molecule, or into separation of those atoms by breaking the chemical bond between them.

So, in reality, the benefits of a nuclear rocket vs a chemical rocket need to be calculated between 2 actual designs, one for each, to determine the real difference for specific mission requirements.

Thanks for that very thorough explanation.

 

[newtons_laws]

The efficiency ("specific impulse" abbreviated Isp) of a rocket motor is the amount of thrust per unit mass of the propellant used per unit time.

So, for a given total mass of propellant, the lighter exhaust particles provide more total thrust over the same period of time that the rocket engine runs.

Precisely, basic rocket propulsion theory.